In general, a power steering system for an automotive vehicle comprises a servomotor which is controlled by the steering wheel for turning the dirigible wheels. Hydraulic servomotors are conventionally used and it is also known to utilize an electric servomotor.
In conventional vehicles, it is desirable for the dirigible wheels of the vehicle to automatically return to the straight ahead position after the driver has executed a turn and releases the steering wheel. In the conventional system, this is realized as the result of the geometry of the steering system. In particular, with the vehicle moving forward and the front wheels turned, the road forces acting on the front wheels tend to turn the front wheels back to the straight ahead position. This restoring force which originates from the reaction of the front wheels with the road surface, is transmitted through the steering linkage to the steering gear. Rotation of the steering gear to the straight ahead position drives the steering shaft and steering wheel to the straight ahead position. In power steering systems without a clutch to disengage the servomotor it also drives the servomotor. In the case of an hydraulic servomotor the drag forces are compensated by changing the steering geometry to increase the restoring force. However, in the case of an electric servomotor, the resisting force due to inertia and friction in the servomotor may be substantial in relation to the restoring force. To minimize the cost and weight of the servomotor it is desirable to use a large gear reduction ratio. This increases the effects of inertia and friction. Without special measures, the resisting force or drag imposed by the electric servomotor has the effect of slowing the return of the front wheels to the straight ahead position. Copending application Ser. No. 597,077, filed Apr. 5, 1984 by Peter Norton for Coupling For Drive Shaft and Servomotor Driven Shaft With Uncoupled Neutral Position discloses a clutch that disengages to eliminate servomotor drag when the driver is not applying torque to the steering wheel. However, under certain circumstances, this clutch can reengage at times when the servomotor speed is substantially different from the servomotor speed required to achieve synchronization with the steering mechanism and reengagement of the clutch at such times results in a clash which may be undesirable. This invention discloses a means for maintaining servomotor speed in approximate synchronization with steering mechanism speed so this clashing is minimized. U.S. Pat. No. 4,577,716 granted Mar. 25, 1986 to Peter Norton for Servo Steering System discloses a system which reduces or eliminates the resistance of the servomotor to the return of the dirigible wheels to the straight ahead position in servo steering systems in which there is no clutch or the clutch does not disengage the servomotor while the steering wheel is freely returning to the straight ahead position.
A general object of this invention is to provide a control system for reducing the speed difference between the servomotor and the steering gear and to include this capability in a power steering system in a neat and compact structure and to overcome certain disadvantages of the prior art.